1. Field of the Invention
The present invention generally relates to electrical connectors, and more particularly to an improved type of connector known as a twist-on or wire-nut connector.
2. Description of the Prior Art
Twist-on connectors (commonly referred to as spring connectors) are well-known in the art. One of the earliest patents disclosing this type of connector is U.S. Pat. No. 2,656,204 issued to J. Blomstrand on Oct. 20, 1953. The Blomstrand connector basically comprises a helically coiled spring, into which twisted wire ends are inserted. As the wire ends are inserted, the coil spring expands slightly and resiliently grips the wire ends. Improved versions of the twist-on connector typically include a hard, electrically insulative shell surrounding the coil spring (such as that shown in U.S. Pat. No. 3,075,038 issued to W. Schinske on Jan. 22, 1963), and the shell often includes integral wing portions (also referred to as ears, tabs, or fins) which allow the user to exert a greater torque while twisting the connector over the wire ends. Minnesota Mining and Manufacturing Company (3M), assignee of the present invention, markets a line of twist-on connectors under the trademarks Scotchlok, Hyflex and Ranger.
One disadvantage inherent in nearly all of the prior art twist-on connectors is the limited range of wire diameters (or absolute number of wires) which the connector can accommodate. As recognized in Underwriters Laboratories' standard 486C for twist-on connectors, this limitation is primarily related to the thickness of the wire insulation. Although a portion of this insulation is removed to allow the wire ends to be twisted together, the insulation must still enter at least slightly into the connector for safety reasons; in other words, no portion of the bare wires should be visible or accessible once the connection is made, to prevent the possibility of a short circuit or other electrical hazard.
FIG. 7 of the Schinske patent suitably illustrates this problem. In many cases, there is sufficient room within the main body of the connector shell (i.e., within the coil spring) to receive multiple wires, but this room often cannot be fully utilized due to the crowding of the wire insulation at the opening of the rigid skirt of the connector. This construction necessarily results in the wasteful use of additional connectors (sometimes requiring "daisy" chains), and in wasted time on the part of the craftsperson making the electrical connections.
One prior art connector which addresses this limitation is shown in U.S. Pat. No. 2,890,266 issued to E. Bollmeier on Jun. 9, 1959. That connector utilizes a metal shell surrounding the coil spring, and an elastic sleeve which surrounds the shell and forms a skirt at the opening of the connector. Bollmeier, however, presents additional problems not present in other prior art connectors. Specifically, it is difficult to exert any torque on the inner metal shell since the sleeve tends to slide around the shell as the connector is twisted over the wires. This drawback may, in some instances, be critical since it affects the integrity of the electrical connection and the pullout force required to remove the wires from the connector.
It is also unproductive to incorporate the wings or tabs of other twist-on connectors into the Bollmeier device since the wings would be integrally formed with the sleeve, which would just exacerbate the tendency of the sleeve to slide around the inner shell. As a further result of the foregoing drawbacks, a user of the connector is required to exert a greater gripping force on the connector, which can deleteriously result in deformation of the inner metal shell and the coil spring. Thus, use of this type of twist-on connector is limited to small wire sizes which require relatively low twisting force to secure the wire bundles. It would, therefore, be desirable and advantageous to devise a twist-on connector having a flexible skirt or opening similar to Bollmeier, but which additionally provides enhanced gripping action and greater torque application, to insure a secure wire connection, and which further accommodates a wider range of wire gauges.